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174 VIII. SOUND, SOUND RECORDING, AND CHARACTERISTICS
a radio receiver, while 75 db represents the highest intensity under similar conditions. Figure 27 shows the effect of intensity upon perceived range for music under ideal laboratory conditions (in the absence of noise). These are the hearing contours for an average listener (median population) and for a critical listener (the 5% with most acute hearing) in the absence of noise.
Influence of Noise
Noise is a very variable element in the reproduction of 16-mm sound films. Not only is there an appreciable variation in the amount of recorded noise from one original film to another, and an additional noise variation from one print to another due to processing, but there is also a variation in projection, since the extraneous noise present in the reproduction room is also variable. There are a number of factors that influence the noise present and its masking effect upon the desired sound :
(1) The number of people in the audience;
{2) The attention which the audience accords the projected film — as measured by the amount of noise that the audience itself produces;
(3) The "liveness" of the auditorium or space where the film is projected;
(4) The noise level of the sound system (amplifier, photocell, exciter lamp, etc.) of the projector;
(5) The noise from the film itself;
(6) The noise recorded and printed upon the film; and
(7) The noise made by the projector itself.*
In addition to these sources of noise, there are the more obvious sources such as electric fans, street traffic, bells, low-flying aircraft, and even the blaring of nearby radio receivers. In a broad sense, noise may be considered the summation of all acoustic disturbances that tend to mask or otherwise reduce the perception of the desired sound intelligence.
Statistical studies of the masking effect of noise upon reproduction at the noise intensities encountered in homes show a still further shortening of the frequency range at both ends of the spectrum. A very quiet residential noise level is 33 db, while the average residential noise is 43 db. Figure 28 shows the loss in frequency range of the average listener under the 43-db noise condition, and Figure 28A shows the similar loss for the critical listener under the relatively quiet 33-db noise condition. * One of the most serious sources of noise is the whirring of the high-speed blower used on the conventional projector for lamp cooling. High-speed blowers are small but make much noise ; low-speed blowers, while quiet, are large in size.